Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. Within the United States, cellular service is offered not only by dedicated cellular service providers, but also by the regional Bell companies, such as U.S. West, Bell Atlantic and Southwestern Bell, and the national long distance companies, such as AT&T and Sprint. The enhanced competition has driven the price of cellular service down to the point where it is affordable to a large segment of the population.
Wireless subscribers use a wide variety of wireless devices, including cellular phones, personal communication services (PCS) devices, and wireless modem-equipped personal computer (PCs), among others. The large number of subscribers and the many applications for wireless communications have created a heavy subscriber demand for RF bandwidth. To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
Although TDMA-based wireless networks were developed on a large scale first, CDMA-based wireless networks are now also widely used. CDMA systems divide the RF spectrum into a number of wideband digital radio signals. Each digital radio signal carries several different coded “channels”. Each coded channel is distinguished by a unique pseudo-random noise (PN) code used by the mobile station and/or the base station. In a CDMA receiver, the coded channels are decoded by a signal correlator that matches PN sequences.
Some coded channels are used as data traffic channels to transport subscriber voice and/or data signals, while other coded channels are used as control, or “overhead,” channels, including Pilot, Synchronization, Paging and Access channels. In some systems, one or more of the Pilot, Synchronization, Paging and Access channels may be combined into a single channel.
When a mobile station accesses a base station in a CDMA wireless network via the overhead channels (each of which has a unique PN code), the network assigns the mobile station to a data traffic channel (which has a different PN code than the overhead channels) on which the mobile station exchanges voice or data traffic with another party (including another mobile station), a data terminal, a fax machine, or the like. Typically, the coded overhead channels and the coded data traffic channels used by the mobile station and the base station are on the same RF carrier frequency. Advantageously, in many CDMA networks, the overhead channels and the data traffic channels are on the same RF carrier frequency in all cells (i.e., base station coverage areas) in the networks.
Each data traffic channel and overhead control channel of a base transceiver station constitutes a “resource” of that base transceiver station. Ultimately, the channel resources of a base transceiver station are limited by the practical restraints imposed by the hardware in the base transceiver station. Thus, a base transceiver station is limited to servicing a certain number of data traffic channels at any one time. This limit is usually determined by the number of channel transmitter/receiver elements that are built into the BTS. If a BTS contains fifty (50) channel transceiver elements that are reserved for data traffic channels, then the BTS is limited to handling a maximum of 50 two-way communication links with 50 mobile stations. If a fifty-first mobile station tries to access the base transceiver station via its access channel, the base transceiver station will refuse the access request until one of the fifty data traffic channels becomes free.
The limitations on the number of data traffic channels served by a base transceiver station also affect the failure recovery capabilities of a base transceiver station. If a hardware or software failure occurs in a base transceiver station, an overhead channel can be lost (i.e., transmission failure) within the corresponding cell site. If the failure occurs on the Paging, Synchronization, or Access channels, the existing calls within the cell site are not lost. However, the failed base transceiver station (BTS) becomes inaccessible and no new calls can be established. If the Pilot channel is lost, not only will the BTS become inaccessible, but all existing calls are dropped within a period of only a few seconds.
To deal with the failure of an overhead channel, conventional wireless systems initiate an overhead channel switchover operation. When a switchover occurs, the BTS reconfigures an unused data traffic channel to operate as an overhead channel by using the same PN code and frequency range used by the failed overhead channel. This is an adequate solution provided that the BTS is not operating at full capacity, such that all data traffic channels are already in use. If no data traffic channel is available, the BTS drops an established call in order to free up a data traffic channel that can be reconfigured as an overhead channel. This is done even if the dropped call is a “911” emergency call or an important business or personal call. In any event, the dropped call is a loss for the consumer and the service provider.
To overcome the problems associated with dropping an established call in order to reconfigure an overhead channel, conventional wireless systems sometimes reserve a “hot” standby channel element for each overhead channel. Unfortunately, this results in a loss of, for example, two to four data traffic channels, depending on how many overhead channels are used to carry the Pilot, Synchronization, Paging and Access signals.
For example, if the Pilot, Synchronization, Paging and Access signals are carried in four separate channels, four standby channels are needed and four data traffic channels are lost. If the Pilot and Paging signals are carried in separate overhead channels and the Synchronization and Access channels are combined in the same overhead channel, then three standby overhead channels are needed and three data traffic channels are lost.
There is therefore a need in the art for a CDMA wireless network that more efficiently utilizes the channel resources of the base transceiver stations in the network in order to serve the greatest number of mobile stations possible. There is also a need for a CDMA wireless network that suffers minimal performance degradation upon the occurrence of a failure in an overhead channel. In particular, there is a need for a CDMA wireless network that minimizes the risk of dropping an existing call in order to reconfigure a data traffic channel as an overhead channel. More particularly, there is a need for a CDMA wireless network that minimizes the risk of dropping an existing call in order to reconfigure a data traffic channel and which eliminates or reduces the need for standby channels.